Hayrteu.] '*'^'^ [May 6, 



This species bears a striking resemblance to Petalodus acuminatus, 

 Agassiz, from the mountain limestone of Europe; but, at the same time, 

 it possesses characters which readily distinguish it from that species. The 

 present species differs mainly in the more tapering root, the coronal band 

 upon the inner face is more strongly curved downwards in the middle, 

 and the crown is relatively higher. This species was originally described 

 from the Upper Coal Measures of central lUinois. I have seen a fine 

 specimen of the same species in the collections of the Museum at Cam- 

 bridge, from similar horizons in southwestern Indiana, and also from the 

 Upper Coal Measures of central Iowa. 



Form, and Loc. — Upper Coal Measures. 



Genus PEMPRISTIS, Agassiz (ms). 



Gen. char. — Teeth small or of medium size, possessing the general char- 

 acteristics of the Petalodonts. Crown compressed, acuminate, serrate, 

 more or less curved laterally; extremities on the inner face connected by 

 a raised transverse shoulder, in which the crown terminates below and 

 which gives rise to a more or less profound- coronal cavity, Eoot well de- 

 veloped, entire, as in Petalodus. The surfaces of the crown and coronal 

 cavity are covered by a dense and highly polished layer of ganoine, which 

 forms an imbricated band at the base. 



The above generic designation was suggested by Prof. Agassiz, for the 

 reception of a group of peculiar teeth, of which we have at least two rep- 

 resentative species — that of F. semicircularis being regarded as the type. 

 These forms certainly possess features which are widely at variance to the 

 typical species of the genus GtenoptycMus, as represented by G. apicalis, 

 Agassiz; and in their description of the following species, Messrs. New- 

 berry and Worthen have also referred to the remarkable characters which 

 distinguish it from the typical species of GtenoptycMus. The central cor- 

 onal cavity and the prominent transverse ridge in which the root is ter- 

 minated above on the posterior aspect, are peculiarities which do not 

 appear in any of the numerous other genera comprised in the groups of 

 Petalodonts. 



The genus is Carboniferous, ranging from the Subcarboniferous to the 

 Upper Coal Measures inclusive. 



Peripkistis semicircularis. 



Bef. and 8yn. — GtenoptycMus semicirctdaris, Newberry and Worthen, 

 Geol. Illinois, Vol. II, p. 72; PI. IV, Figs. 18, 18a, 18b. 



Description. — Tooth small, broadly obovate in outline, crown much 

 compressed and strongly curved laterally, giving the crest a semicircular 

 outline viewed from above; cutting edge divided into seven to hine denti- 

 culations, the median lobe strcpigest, lateral ones gradually decreasing in 

 size toward the lateral extremities, where they are scarcely relieved from 

 the edge; the calcigerous tubes slightly diverge on nearing the edge, pro- 

 ducing a minute radiated striation of the denticulations like that observed 

 in the even crest of Petalodus, and when the crown is much worn the 

 surface is finely punctate ; outer face of crown very low in proportion to 



